Petrophysical properties of the Kylylahti Cu-Au-Zn sulphide mineralization and its host rocks

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Title: Petrophysical properties of the Kylylahti Cu-Au-Zn sulphide mineralization and its host rocks
Author: Luhta, Tuija
Other contributor: Helsingin yliopisto, Matemaattis-luonnontieteellinen tiedekunta
University of Helsinki, Faculty of Science
Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten
Publisher: Helsingin yliopisto
Date: 2019
Language: eng
Thesis level: master's thesis
Discipline: Geofysiikka
Abstract: This work presents a new set of petrophysical laboratory measurements from Kylylahti, a Cu-Au-Zn mine in the Outokumpu mining district, in the eastern Finland. Results are discussed and compared to earlier petrophysical data from the area. The study was aimed to provide a solid base for accurate interpretation of already existing geophysical exploration data and to new seismic data collected during the COGITO-MIN (COst-effective Geophysical Imaging Techniques for supporting Ongoing MINeral exploration in Europe) project. The sample set covered the most common rock types found in Kylylahti. A small set of samples represented sulphide mineralizations from several mining sites in the Outokumpu district. In the area, ophiolitic ultra-mafic massifs consisting of Outokumpu assemblage rocks, are embedded in Kalevian sediments, black schists and mica schists. Several massifs, Kylylahti being one of them, contain polymetallic (Cu-Co-Zn-Ni-Ag-Au-Cd-Sn-As-Se-Mo) massive, semi-massive or disseminated sulphide mineralizations. The petrophysical parameters measured were density, seismic P-wave velocity, porosity, magnetic susceptibility, intensity of remanent magnetization, inductive resistivity, galvanic resistivity and chargeability. Additional parameters calculated from the measurements were seismic impedance, Königsberger (Q) ratio and induced polarization (IP) estimates. Density data divides the Kylylahti rocks in three categories: 1) Massive and semi-massive sulphide mineralizations with an average density of 3750 kg/m 3 , 2) Outokumpu assemblage rocks with densities close to 3000 kg/m 3 and 3) Kalevian rocks with densities a bit under 2800 kg/m 3 . Sulphide disseminations are common in Outokumpu assemblage carbonate-skarn-quartz rocks and black schists elevating the densities when abundant. The average P-wave velocities for almost all Outokumpu assemblage rock types are a bit over 6 km/s. Soap stones, mica schists and black schists have lower P-wave velocities, around 5.5 km/s. Porosity of the samples was very low overall. Most of the Kylylahti rocks belong to paramagnetic group (susceptibilities under 2000 μSI). Serpentinites and tremolitic calc-silicate rocks (TRECS) belong to strongly magnetic group as well as samples rich in disseminated sulphides. Low Q ratios reveal that magnetic mineral in serpentinites and TRECS is coarse-grained magnetite. Samples with disseminated sulphides have high Q ratios, thus the disseminations are mainly monoclinic pyrrhotites. Both Kylylahti sulphide mineralizations and black schists are conductive as well as rocks rich in dissiminated sulphides. The rocks containing disseminated sulphides have high IP estimates. Conductivity of black schists is due to graphite and to some extent due to disseminated sulphides. Physical properties of the ore samples from different mining sites reveal the differences in their mineralogy, mainly their changing proportions of pyrite, pyrrhotite and magnetite. The differences are due to metamorphic zoning in the Outokumpu district; the degree of metamorphism becomes higher when going from east to west or from surface to depth. Recommended parameters, densities and P-wave velocities for seismic modelling in Kylylahti are given. Based on the results, the sulphide mineralizations should produce a detectable reflection against any background due to their high density. Also the other Outokumpu assemblage rocks have a clear contrast against the mica schists and black schists. Soap stones are an exception. The contact between Kalevian rocks and soap stones is hardly reflective at all, whereas soap stones in contact with other Outokumpu assemblage rocks form a reflecting contact.

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